1. Field of the Invention
The invention relates to a method for programming a ROM cell arrangement on a semiconductor. In particular, the invention relates to producing or programming a ROM cell arrangement that includes decoders.
2. Description of Related Art
Read-only memories, also known as ROMs, are used to store data in many electronic systems. Generally, data in ROM are written in permanently in digital form and are realized as silicon circuits which are constructed in an integrated manner on a basis of a semiconductor material, particularly silicon, and in which MOS transistors are preferably used as memory cells.
In the readout, the individual memory cells are selected via the gate electrode of the MOS transistors, which is connected to a word line. The input of each MOS transistor is connected to a reference line; the output is connected to a bit line. In the read process, it is ascertained whether or not current is flowing through the transistor. The logical values one ("1") and zero ("0") are assigned to the stored data, accordingly.
Technically, the storing of 0 and 1 in these ROMs is effected in that, in memory cells in which the logical value that is assigned to the condition "no current flow through the transistor" is stored, a MOS transistor is not produced, or a conductive connection to the bit line is not realized. Alternatively, corresponding MOS transistors can be realized for the two logical values. The transistors have different inception voltages in the channel region due to different implantations. Such a silicon memory has an essentially planar construction with a minimal surface area per memory cell, yielding a typical memory density of approximately 0.14 bit/.mu.m.sup.2 in today's one-time technology.
The invention relates primarily to the production of one-time electrically programmable read-only memories, what are known as OTP memories ("One-Time Programmable Memory") in which the gate dielectric usually has an ONO forming material but it can also be used in principle in multiple programmable read-only memories ("MTP-ROMs") in which the gate dielectric has a gate oxide (see FIG. 1).
During the programming process of a memory cell in which very high voltages are applied (e.g. known as the Fowler-Nordheim mechanism), the neighboring cells of the affected cell, which are organized as a matrix in the cell field, must be protected. This typically occurs by applying a blocking voltage at the bit lines of the cells to be protected and applying a protecting voltage at the word lines, in order to keep the voltages at these neighboring cells low. The feeding of the voltages occurs by decoders which are equipped with corresponding high-voltage switches.
A word decoder usually functions as a "1 out of n" decoder during the programming process. The word decoder connects one line to the higher programming voltage, while all the remaining decoder outputs remain at the protective voltage. A bit decoder likewise functions as a "1 out of n" decoder during the programming process. The bit decoder connects one line to the mass potential, while the other bit lines remain at the blocking potential. A blocking decoder typically functions as a "n-1 out of n" decoder and connects those bit lines to the blocking potential at which an information value will not be programmed. The one remaining line is kept at mass potential by the bit decoder. The previously known methods for programming an OTP memory require relatively expensive decoder circuits and are nevertheless relatively slow.
U.S. Pat. No. 4,858,194 teaches a non-volatile semiconductor memory in which the bit lines and word lines are precharged to a fixed blocking voltage prior to the writing, and only in the writing are a relevant bit line and a relevant word line brought to a respective potential depending on the information to be written in.